Always set a clip on the painter in QGraphicsView.

This allows items to check painter->clipRegion() to find out what the
imposed clip is, in order to cull elements that don't need to be
painted. This is an alternative approach to getting the same
information from QStyleOptionGraphicsItem::exposedRect. It's better
because it's a pull operation, but it's slightly worse because it
doesn't include the complete system clip, and because QRegion has
integer resolution only (whereas QGraphicsItem's coordinate uses
qreal.

A better approach may be to access QPainter's combined clip region;
this option is open for future versions of Qt.

Original patch by Warwick (which is why he's on reviewed-by), but the
patch was modified to operate in device instead of logical coordinates.

Reviewed-by: jasplin
Reviewed-by: Warwick Allison

2 files changed, 75 insertions, 0 deletions

diff --git a/src/gui/graphicsview/qgraphicsview.cpp b/src/gui/graphicsview/qgraphicsview.cpp
index c88f678..d1dd26f 100644
--- a/src/gui/graphicsview/qgraphicsview.cpp
+++ b/src/gui/graphicsview/qgraphicsview.cpp
@@ -3252,10 +3252,13 @@ void QGraphicsView::paintEvent(QPaintEvent *event)
 
     // Determine the exposed region
     d->exposedRegion = event->region();
+    if (d->exposedRegion.isEmpty())
+        d->exposedRegion = viewport()->rect();
     QRectF exposedSceneRect = mapToScene(d->exposedRegion.boundingRect()).boundingRect();
 
     // Set up the painter
     QPainter painter(viewport());
+    painter.setClipRect(event->rect(), Qt::IntersectClip);
 #ifndef QT_NO_RUBBERBAND
     if (d->rubberBanding && !d->rubberBandRect.isEmpty())
         painter.save();
diff --git a/tests/auto/qgraphicsview/tst_qgraphicsview.cpp b/tests/auto/qgraphicsview/tst_qgraphicsview.cpp
index f07453c..9c6aa39 100644
--- a/tests/auto/qgraphicsview/tst_qgraphicsview.cpp
+++ b/tests/auto/qgraphicsview/tst_qgraphicsview.cpp
@@ -217,6 +217,7 @@ private slots:
     void update();
     void inputMethodSensitivity();
     void inputContextReset();
+    void defaultClipIntersectToView();
 
     // task specific tests below me
     void task172231_untransformableItems();
@@ -3692,6 +3693,77 @@ void tst_QGraphicsView::inputContextReset()
     QCOMPARE(inputContext.resets, 0);
 }
 
+class ViewClipTester : public QGraphicsView
+{
+public:
+    ViewClipTester(QGraphicsScene *scene = 0)
+        : QGraphicsView(scene)
+    { }
+    QRegion clipRegion;
+
+protected:
+    void drawBackground(QPainter *painter, const QRectF &rect)
+    {
+        clipRegion = painter->clipRegion();
+    }
+};
+
+class ItemClipTester : public QGraphicsRectItem
+{
+public:
+    ItemClipTester() : QGraphicsRectItem(0, 0, 20, 20)
+    {
+        setBrush(Qt::blue);
+    }
+    QRegion clipRegion;
+
+    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0)
+    {
+        clipRegion = painter->clipRegion();
+        QGraphicsRectItem::paint(painter, option, widget);
+    }
+};
+
+void tst_QGraphicsView::defaultClipIntersectToView()
+{
+    QGraphicsScene scene;
+    ItemClipTester *tester = new ItemClipTester;
+    scene.addItem(tester);
+
+    ViewClipTester view(&scene);
+    view.setAlignment(Qt::AlignTop | Qt::AlignLeft);
+    view.setFrameStyle(0);
+    view.resize(200, 200);
+    view.show();
+    QTRY_COMPARE(QApplication::activeWindow(), (QWidget *)&view);
+
+    QRect viewRect(0, 0, 200, 200);
+    QCOMPARE(view.clipRegion, QRegion(viewRect));
+    QCOMPARE(tester->clipRegion, QRegion(viewRect));
+
+    view.viewport()->update(0, 0, 5, 5);
+    view.viewport()->update(10, 10, 5, 5);
+    qApp->processEvents();
+    viewRect = QRect(0, 0, 15, 15);
+    QCOMPARE(view.clipRegion, QRegion(viewRect));
+    QCOMPARE(tester->clipRegion, QRegion(viewRect));
+
+    view.scale(2, 2);
+    qApp->processEvents();
+
+    viewRect.moveTop(-viewRect.height());
+    viewRect = QRect(0, 0, 100, 100);
+    QCOMPARE(view.clipRegion, QRegion(viewRect));
+    QCOMPARE(tester->clipRegion, QRegion(viewRect));
+
+    view.viewport()->update(0, 0, 5, 5);
+    view.viewport()->update(10, 10, 5, 5);
+    qApp->processEvents();
+    viewRect = QRect(0, 0, 8, 8);
+    QCOMPARE(view.clipRegion, QRegion(viewRect));
+    QCOMPARE(tester->clipRegion, QRegion(viewRect));
+}
+
 void tst_QGraphicsView::task253415_reconnectUpdateSceneOnSceneChanged()
 {
     QGraphicsView view;